Fiber optic sensors employing dual-core fibers have a number of uses in such devices as smark skins, hydrophones, magnetometers, alignment determination and interferometric sensors. In a typical dual-core cross section a pair of single-mode cores are located within the cladding at positions nominally equidistant from the central axis of the fiber. There are many applications, such as interferometric sensors in which it is required that light be launched into both cores simultaneously and that phase information be retrieved from these cores. Because of the non-symmetrical geometry, the alignment required for accomplishing this is difficult. In the prior art this has been accomplished employing bulk optics, such as converging lens and precision 3-axis positioning stage. However, since the bulk optics are unstable, large in size and expensive, their usefulness is limited to instruments constructed as laboratory prototypes and they are unsuitable for many practical sensors, particularly where size and expense factors require that the entire device be fabricated from optical fibers. Thus the requirement for bulk optical components has ruled out the use of the dual-core fiber sensors in small, inexpensive applications.
This non-rotationally symmetric aspect of dual-core fibers makes it substantially impossible to splioe or connect them to a single core/single mode fiber, since highly accurate core to cladding concentricity (typically less than 1 micron) is required. Since diode lasers, light emitting diodes, integrated optics and photodetectors are now available with a fiber pigtailed configuration, the ability to splice this pigtail with low losses into dual-core fibers is highly desirable.